Microwave components have traditionally been mounted on hard substrates. By hard substrate is meant here a board comprised of ceramic material. Electrical metal conductors have been provided on the substrate. In the case of microwave technology, the electrical conductors are designed to constitute electrical components in themselves. Naturally, the conductors also serve to connect components mounted on the substrate. An undesired function of the electrical conductors is that they also function as antennas in microwave applications. Consequently, the electrical components must be shielded/screened against interference, so as not to transmit signals or receive interference signals originating from outside sources. This is effected by encapsulating and hermetically sealing the substrate. Electrical conductors are drawn through the wall of the capsule for connecting the capsule to other devices and equipment.
Electrical conductors are made of metal, whereas the capsule is sometimes made of ceramic material, similar to the substrate. Metal is subjected to thermal expansion, whereas the ceramic capsule will not expand in response to higher temperatures, at least not to a significant extent in comparison with the metal. The differences between the coefficients of thermal expansion of the materials means that the transit or lead-through in the capsule wall is difficult to seal hermetically.
U.S. Pat. No. 5,109,594 teaches a solution to this problem, with the aid of a bi-metal expansion collar fitted in a hole in the capsule wall. The electrical conductor is then inserted through the collar. The collar forms a packing which seals between the electrical conductor and the hole-defining walls. The collar is made of two materials that have coefficients of thermal expansion which lie between that of the metal conductor and that of the capsule wall-the one close to the coefficient of thermal expansion of the capsule wall and the other close to the coefficient of thermal expansion of the metal. The two materials of the collar are separated axially. That end of the collar whose coefficient of thermal expansion lies close to that of the capsule is fastened to the capsule, and the opposite end of said collar is welded to the metal conductor.
In recent times, the technology concerning the manufacture of hard substrates has been developed so as to enable the boards to be provided with several layers of conductors in manufacture, in addition to surface layers. A substrate which will allow this is designated an LTCC substrate (Low Temperature Cofired Ceramic), while another functional substrate in this respect is the HTCC substrate (High Temperature Cofired Ceramic). Such substrates are referred to as hard multilayer substrates in the following text. Conductors in the various layers are connected through vias. A hard multilayer substrate produced in accordance with this development will function as a printed circuit board. Although hard substrates are suitable for use with microwave electronics, the use of hard multilayer substrates is not restricted solely to microwave electronics.
Encapsulation of layers of conductors within the hard multilayer substrate can be avoided, by screening the conductors against interference through the medium of earth planes on both sides. Although encapsulation is still required in respect of surface mounted components and conductors, it is not necessary to encapsulate the entire hard multilayer substrate, but solely the part or parts that carry microwave components or conductors on the surface plane. The encapsulated parts of the hard multilayer substrate are connected electrically via conductors in layers within the substrate.
In recent times, it has also been possible to manufacture hard multilayer substrates with surfaces measurements that correspond with A4 sizes. Hard multilayer substrates are brittle as distinct from printed circuit boards that include plastic instead of ceramic as insulating layers. It is therefore not possible to rivet electrical connectors to the hard multilayer substrate. Components and electrical connectors to be mounted on the substrate must therefore be surface mounted. One difficulty in respect of surface mounted components and electrical connectors is one of ensuring that the components and connectors will remain firmly seated when the temperature varies. This difficulty is due to the fact that the materials in the surface mounted components and connectors have thermal expansion coefficients that are different to that of the printed circuit board. Microwave electronics often operate with high powers and therewith high heat generation.